Many generations ago, there was a population of Grey Wolves. They had many different color variations, such as brown, grey, and white. They live in the mountainous forests of Eurasia and North America. Wolves usually hunt small rodents, or every once in a while, a deer or ox. They had no predators in their environment and they were the top tier hunters
Slowly but surely, the wolves environment started changing. The rain turned to snow, and the ground turned white, making the wolves of darker colors, such as brown and grey, not be able to sneak up on their prey. The grey and brown wolves would eventually starve, and most likely not live long enough to reproduce and pass on their color
Eventually, after about 100 generations of dark wolves dying and light ones living, the entire population of wolves were light enough where their prey would stuggle to see them coming. Dark colors were not adaptive in this environment, and they would not survive.